Background: The "outcomes movement" is growing, and investigators are increasingly using health status instruments to evaluate the outcomes of randomized clinical trials. One popular approach utilizes instruments (such as the Medical Outcomes Study Short Form-36 [SF-36]) that provide a profile of scores. Another approach utilizes instruments (such as the Quality of Well-being (QWB) scale) that provide a single measurement While there have been many evaluations comparing the relative strengths of these two instruments, to date these evaluations have not included a long-term clinical trial and these relative strengths have been compared only quantitatively, not qualitatively. The NIH-funded Stroke Prevention and Atrial Fibrillation (SPAF) III project is a long term clinical trial that compares two methods of stroke prophylaxis in patients with nonvalvular atrial fibrillation. The SPAF m project includes the estimation of health-related quality of life using the SF-36. The proposed project adds the assessment a quality of life using QWB interviews to the SPAF m project Objectives/Aims: 1) Evaluate the relative strengths of the profile approach (as exemplified by the SF 36) and the single score approach (as exemplified in the QWB) within the context of a long term clinical trial. These strengths will be assessed using both quantitative and qualitative methods. Specifically, Ia) Assess how health-related quality of life (HRQOL) as estimated by the QWB correlates with the HRQOL estimated by a simultaneously administered SF-36, lb) Evaluate the degree to which each instrument is limited by "floor" and "ceiling" effects, and 1c) Evaluate how the product of the two instruments are evaluated by four different groups: researchers, clinicians, patients, and policy makers. 2) Assess how HRQOL as estimated by the QWB correlates with other appropriate factors evaluated by SPAF. This assessment will include the predictive power of these factors for HRQOL over time. 3) In addition we propose to compare the QALYs produced by the subjects in the two SPAF m treatment arms. Design: SPAF III is a multicenter, randomized trial of aspirin plus low- dose warfarin vs. standard dose warfarin that began in the Fall of 1993. The proposed project will perform QWB interviews of the SPAF subjects over the phone. The SPAF III project obtains a consent for these interviews at the time of entry, and baseline QWB interviews have been obtained with other funding. Follow-up QWB interviews will occur every 6 months. Standard quantitative and qualitative methods will be used to evaluate the relative strengths of the SF-36 and the QWB.